In Long Term Evolution (LTE), a short transmission time interval (TTI) such as 1 [ms] is employed to realize a higher data rate. A shorter TTI shortens the round trip time (RTT) required for retransmission control and reduces system delay.
On the other hand, in a system with a short TTI, for example, a mobile station (user equipment (UE)) located at a cell end cannot sufficiently ensure energy for transmission because of limited transmission power. Therefore, retransmission may frequently occur even in the case of a smaller packet of Voice over Internet Protocol (VoIP), etc. In this case, for example, a delay of 8 [ms] (for RTT) occurs for each retransmission. Particularly, an application requiring real-time performance such as VoIP is vulnerable to such a delay and a mechanism for reducing delay is desired.
LTE employs TTI bundling as a technique against such a delay (see, for example, Japanese Laid-Open Patent Publication No. 2009-253981). The TTI bundling is a technique of continuously transmitting the same packets for several TTIs and combining the packets on the reception side to achieve improved reception quality.
However, the conventional technique has a problem that communication efficiency cannot sufficiently be improved.
For example, 3rd Generation Partnership Project (3GPP) defines that continuous transmission is executed for four TTIs, i.e., the same number as that of types of redundancy versions (RV). However, transmission for four TTIs is not always necessary and transmission for first several TTIs may ensure sufficient reception quality and make a cyclic redundancy check (CRC) OK (error-free) in some cases.
For example, in the TTI bundling with four-TTI continuous transmission, if a first mobile station acquires a CRC result of OK in two TTIs, the first mobile station executes the continuous transmission for four TTIs.
Therefore, a second mobile station having available resources must wait until the first mobile station completes the four-TTI continuous transmission. This is not efficient in terms of resource utilization and power consumption.
On the other hand, in an environment with poor communication channels, transmission for four TTIs cannot ensure sufficient reception quality and retransmission control may occur. Particularly, since a short TTI of 1 [ms] is employed and the same frequency resource is used for the continuous transmission, the TTI bundling is susceptible to burst noise and local deterioration of frequency characteristics. If the retransmission control occurs in the TTI bundling, resources and transmission power for four TTIs are further used, thereby increasing the resources used, power consumption, delay, etc.
In the TTI bundling, a base station feeds back ACK or NACK after receiving all the bundled signals, for example. Therefore, if retransmission occurs in the TTI bundling, retransmission control takes more time as compared to when the TTI bundling is not performed.
Since the same frequency resource is ensured for multiple TTIs in the TTI bundling, flexibility in resource allocation drops. Therefore, for example, if resources cannot be ensured for multiple continuous TTIs, transmission is postponed. Particularly, if more mobile stations are accommodated by a base station, the probability that mobile stations contend for resources increases and therefore, frequent suspension of transmission can be expected.